1. Field of the Invention
The present invention relates to a surface light source device to be applied to a backlighting of a liquid crystal display or the like, and more particularly to a surface light source device having a characterizing feature in a holder for a straight light source (a straight lamp such as a straight discharge tube) for supplying a light guide with light.
2. Related Art
FIGS. 5 and 6 are cross sectional views showing a general structure of a conventional surface light source device. Referring to FIGS. 5 and 6, a surface light source device comprises a straight lamp 1 such as a cold cathode tube, a light guide 2, a diffusive member 3 disposed on an emitting side of the light guide 2, and a reflection member 4 such as a reflection sheet disposed on a side of the light guide 2, which is opposite to the side at which the diffusive member is disposed. The straight lamp 1 is surrounded from its rear side by a lamp holder 11 having a reflection surface inside.
Generally, a discharge tube such as a cold cathode tube is adopted as a straight lamp for use in this type of surface light source device. The discharge tube such as a cold cathode tube has an electrode portion 1a at each of opposite ends thereof, as shown in FIG. 6, and therefore a power of light supply to an incidence end surface 2a of the light guide 2 in the vicinity of the electrode portions 1a is relatively low as compared with the central portion. As a result, an uneven luminance is caused in an emitting surface 2b of the light guide 2.
In an ordinary cold cathode tube, luminance sharply drops in the range of approximately 15% of the full length from the tube ends. FIG. 7 is a graph for showing the drop of luminance, in which the horizontal axis represents a longitudinal position in the tube and the vertical axis represents luminance (nt) of the emitting surface 2b. The origin of the horizontal axis corresponds to one of the opposite ends of the tube. As shown in this graph, in general, luminance sharply drops at positions spaced apart from the opposite ends of the tube by a distance 10-20% of the full length towards the opposite ends and is substantially constant in both portions around the center, each portion occupying approximately 35% of the full length (approximately 70% in total).
As a solution for eliminating the difference of the light-supplying power between the end portions and the central portion, there is known a method of realizing a uniform intensity of the light emitted from the discharge tube by attaching a shading material or a light reflective material to a part of the surface of the discharge tube.
However, according to the above conventional method, as the light emitted from the central portion, which is inherently bright, is suppressed, the total luminance is reduced. Further, as the light reflected by the inner surface of the lamp holder is shielded by the shading material or reflective material attached to the surface of the discharge tube, the efficiency of utilizing the light would be lowered.
Another problem with the conventional method is that the shading material or reflective material pasted on the tube surface tends to be deteriorated or peeled by the heat generated by the discharge tube.
Still another problem is that the luminance of the emitting surface 2b in the vicinity of the side surface 2d tends to be further lowered by the following causes.
A part of the light entered and transmitted in the light guide 2 comes out of the light guide 2 from the side surface 2d. This phenomenon decreases the light emitted from the emitting surface 2b of the light guide 2 in the vicinity of the side surface 2d. The phenomenon results in further drop of luminance of the emitting surface 2b in the vicinity of the side surface 2d when combined with the insufficient light-supplying power in the vicinity of the electrode portions 1a of the straight lamp (discharge tube) 1, as mentioned.
If the side surface 2d of the light guide 2 is made a reflection surface in order to eliminate the leak of light from the side surface 2d, the manufacturing cost of the light guide would increase.
From another point of view, the light guide is usually molded of synthetic resin by an injection molding. In the molding, a gate for pouring resin may be located at the incidence end surface. If the gate is located in such a position, a vestige of a cut-out gate is left in the central portion of the incidence end surface 2a. In the presence of the gate vestige in the central portion of the incidence end surface of the light guide, the light taken from the gate vestige into the guide light is diffused there so that the greater part of the incident light is emitted from the emitting surface 2b immediately near the incidence position.
As a result, the light emitted from the central portion of the incidence end surface 2a becomes relatively intense, thus causing the luminance unevenness.